Abstract: Climate change, and particularly rapid temperature changes which are projected with the greatest certainty, will result in significant hydrologic changes in the Western U.S. in the 21st century including reduced natural storage as mountain snowpack, increased river flow in winter, reduced flow in summer, and increased water temperature. Changes in hydrologic extremes (droughts and floods) are likely to occur. Widespread loss of glacial storage will likely reduce late summer flows in low flow years in sensitive areas. Hydrologic impacts will not be equally distributed, and areas near freezing in mid winter will be the most sensitive to warming related losses of snowpack and resultant streamflow timing shifts. A large number of impact pathways related to engineering design, water resources management, water quality, and ecosystem function are likely to be activated by these hydrologic changes. As a result there is a wide-spread need to incorporate expected changes in climate into long range planning activities related to water at all levels of governance. Achieving this goal will require the development and use of model-based scenarios to replace historic records of important variables such as streamflow, and will also require more sophisticated and flexible approaches to ecosystem protection and restoration, water planning, and engineering design problems related to water.